It is demanded that an automotive internal combustion engine (hereinafter referred to as the engine) exhibit, for instance, satisfactory driveability, emissions performance, and fuel consumption rate. A control device for the engine controls the engine by operating various actuators to meet such demands. When computing an actuator operation amount, the control device uses various models that are obtained by modeling the functions and characteristics of the engine. The models include a physical model, a statistical model, a combination of these models, and various other models. For example, an air model is used as a model for engine control. The air model is obtained by modeling the response characteristics of an intake air amount with respect to a throttle operation. Further, various maps, such as an ignition timing map for ignition timing determination, and a group of maps may also be used for engine control. In addition to such element-level models, a large-scale model obtained by modeling the entire engine may also be used by the control device described, for instance, in JP-A-2009-47102.
Obviously, the higher the accuracy of a model used for computation, the higher the accuracy of actuator operation amount determination. However, on the other side of the coin, the higher the accuracy of the model, the higher the load imposed on computation based on the model. Although the computation capability of the control device has been enhanced year after year, there is a limit to computation capability enhancement. Therefore, from the viewpoint of computational load, models exhibiting very high accuracy are not always applicable to conventional control devices. Especially when the employed model performs computation at each predetermined crank angle, the computational load varies with the engine speed. That is why the details of the model have to be determined with respect to a high engine speed region in which the computational load is high. In other words, it is difficult for the conventional control devices to use highly accurate models from the viewpoint of computational load within the high engine speed region although the computational load is adequately low within a low engine speed region, which is frequently used under normal conditions.